Thermoplastic vulcanizates (TPVs) are a diverse family of rubber-like materials that exhibit properties of both thermoset rubbers and thermoplastic materials. TPVs show superior aging performance and chemical resistance like thermosets but are easily processed and recyclable within the manufacturing process like thermoplastics. TPVs generally comprise dynamically vulcanized rubber, such as ethylene propylene diene terpolymer (EPDM) rubber, in a thermoplastic matrix comprising a thermoplastic polyolefin such as polypropylene (PP). The TPV looks, feels and behaves like rubber but, due to the thermoplastic content, offers simple, flexible part design and manufacturing.
TPVs are produced by compounding the rubber and thermoplastic components in a reactive extruder such that the rubber component is at least partially crosslinked or vulcanized while being dispersed in the thermoplastic phase. To enable crosslinking of the diene-containing rubber component, a curing agent, such as a phenolic resin, is normally added to the extruder, together with stannous chloride, as a curing accelerator, and zinc oxide, as a curing moderator. Both stannous chloride powder and zinc oxide powder are very difficult to handle regarding materials conveying and feed metering. Additionally, stannous chloride is known to cause severe skin burns, severe eye damage, and as a respiratory irritant. In addition, although in powder form, the stannous chloride and zinc oxide must be metered into the reactor in small, but accurately controlled, quantities, which impose serious challenges for the commercial material conveying, and feed equipment employed.
It is known from, for example, British Patent No. 2455981B that tin (II) chloride may be encapsulated by extrusion in a thermoplastic polymer, such as polypropylene, polyethylene or poly (meth)acrylic acid. The resultant encapsulate is reported to provide a safe and readily transported and easily stored form of tin (II) chloride, which can be used for a variety of industrial uses, including the crosslinking of a polymer mixture comprising natural rubber.
Similarly, International Patent Publication No. WO2015/008053A1 discloses stannous chloride entrained in a thermoplastic polymer wherein the stannous chloride is a particulate form of stannous chloride comprising a stannous chloride particle core coated with a layer comprising stannous oxide. The composition is said to provide low acidity in contact with water and more even distribution of the stannous chloride in the resultant product. The composition is reported as being useful in the preparation of natural and synthetic rubbers, particularly when used in a coextrusion process.
In addition, US Publication No. 2013/0041090A1 discloses a method for producing a thermoplastic elastomer composition, the method involving subjecting an ethylene-α-olefin-based copolymer rubber (A) and a polyolefin-based resin (B) in the presence of an alkylphenol resin (C) and a metal halide (D) to dynamic thermal treatment within a melt-kneading apparatus, wherein the metal halide (D) is a powder, and a mixture of a powder of the metal halide (D) and a particle having a volume-average particle diameter of 0.1 μm to 3 mm is continuously fed to the melt-kneading apparatus. The particle having a volume-average particle diameter of 0.1 μm to 3 mm is preferably a polyolefin-based resin particle and the component (D) is preferably stannous chloride. Mixing apparatuses, which are of non-melt type, such as a tumbler mixer and a Henschel mixer, are preferred for combining the component (D) with the polyolefin-based resin particle.
Despite these proposals, to date there appears to have been no disclosure or suggestion of a combined masterbatch of stannous chloride powder and zinc oxide powder for use in the production TPVs. Although the reason for this omission is not clear, mixing of stannous chloride and zinc oxide in a molten polymer above the melt temperature of the polymer has the potential for the generation of many adverse chemical reactions. For example, the following reaction to produce stannous oxide is possible:ZnO+SnCl2→SnO+ZnCl2.Although stannous oxide can exist as a stable bluish black form or a meta-stable red form, it is generally undesirable in a polymer compound because it is known to cause black specks.
According to the present invention, it has now been found that combined masterbatches of stannous chloride powder and zinc oxide powder encapsulated in a carrier polymer, such as EPDM rubber, maintain the white color of the main EPDM rubber and activate phenolic resin curing surprisingly well when used for TPV preparations.